1. Field of the Invention
The present invention relates to a lithium vanadium oxide, to a process for its preparation, and to its use as cathode active material in a lithium battery.
2. Description of the Related Art
The main properties desired for a material intended to be used as cathode active material in a lithium battery are a high specific energy (which is the product of the capacity and the average potential) and a long cycling lifetime. The material Li1+xV3O8, which has been widely studied, makes it possible to obtain a high capacity, of greater than 150 Ah/kg during the first discharges, but this capacity decreases fairly rapidly with the number of charge/discharge cycles. Various processes for the preparation of L1+xV3O8 by reaction of Li2CO3 and V2O5 are described in the prior art. Thus, Hammou, et al. [Electrochim. Acta, 13 (1988), 1719] describe a process for the preparation of lithium vanadium oxide in which the reactants react in the air at 590° C. for 6 h, and the use of the oxide obtained in a Li-polymer battery. However, this temperature is very close to the melting point, the result of which is a sintering of the powder, which has to be milled before being used in the preparation of a composite electrode. The study of the cycling behavior shows a loss in capacity of the order of 15% between the 1st and the 15th cycle. U.S. Pat. No. 6,136,476 discloses a process in which the reactants are heated at a temperature below the melting point, preferably between 350° C. and 550° C., after having mixed the reactants using an air jet disintegrator for the purpose of reducing the size of the particles and of homogenizing the distribution in the size of the grains. Other techniques for the preliminary mixing of the reactants are mentioned, in particular the use of rotary drum mixers, of vibration mills or of bead mills. Chaloner-Gill, et al [J. Electrochem. Soc., 147, (10) 3575-3578 (2000)] describe a process comprising a sequence of stages, namely: milling the mixture of reactants, heating in the air at 585° C. for 16 h, cooling and remilling, second heating in the air at 585° C. for 16 h, and reaction with Li2S. The material obtained is used as cathode active material in a lithium battery and it is recorded that the capacity decreases significantly with the number of charge-discharge cycles. U.S. Pat. No. 5,520,903 discloses a process which consists in milling the reactants for the purpose of mixing them and of reducing the size of the grains, in compressing, in order to form a compact powder, and in then heating the compressed mixture at a temperature of between 580 and 585° C. In this case, the product obtained is an agglomerate of microcrystalline grains fused to one another which has to be milled before being used as electrode material. The milling certainly gives free grains but it also results in the loss of the monocrystalline morphology of rods and crushed polycrystalline grains are obtained. It is recorded that, in a specific embodiment, without compression, the product obtained after heating comprises a not insignificant amount of impurities of the V2O5 type.